Twelve-Week Warfarin Dose Test Noninferior to Four-Week

TUESDAY, Nov. 15 (HealthDay News) -- Assessing warfarin dosing every 12 weeks is noninferior to assessment every four weeks, according to a study published in the Nov. 15 issue of the Annals of Internal Medicine.

Sam Schulman, M.D., Ph.D., from McMaster University in Hamilton, Canada, and colleagues investigated whether warfarin dose assessment every 12 weeks is as safe as assessment every four weeks in patients receiving long-term warfarin therapy. A dosing assessment was performed every 12 weeks in 124 patients, and every four weeks in 126 patients for 12 months. Patients in the 12-week group had blood drawn every four weeks; for two of the three four-week periods, sham international normalized ratios (INRs) within the target range were reported. The primary outcome measure was the percentage of time spent in the therapeutic range. Secondary outcomes included the number of extreme INRs, changes in maintenance dose, major bleeding events, objectively confirmed thromboembolism, and death.

The investigators found that patients in the four-week group spent 74.1 percent of the time in the therapeutic range compared with 71.6 percent for those in the 12-week group (absolute difference, 2.5 percent; P for a 7.5 percent margin = 0.020). Significantly fewer patients in the 12-week group were found with any dose related changes than those in the four-week group (37.1 versus 55.6 percent). No differences were seen in the secondary outcomes.

"Every-12-week dosing was noninferior to every-four-week dosing with respect to the percentage of time spent in the therapeutic INR range, and the prolonged dosing assessment interval seemed safe," the authors write.